Stanford University now has a Youtube channel, with free access to HD video of full courses on everything from dynamical systems to quantum entanglement. More conferences and workshops are videotaping their talks. What are videos online that you think everyone should know about?

I'll seed this with a few answers to presentations that are mostly expository, but what I'm hoping might happen is that this community wiki could turn into a resource to share excellent presentations of new research, as well as a place to learn (or reinforce) background in an unfamiliar area.

I watched the first lecture in this series and find his explanation of Turing machines very awkward. Furthermore, he seems to suggest that circuits are somehow equal in power to TMs, which is not the case (unless there is more to them than he tells). He himself says he does not want to talk about "strange computation models" but rather about "familiar combinatorics" so I'd say these are rather maths, not TCS lectures.
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RaphaelOct 20 '10 at 9:27

Richard Feynman's Messenger Lectures restored, with annotations, by Microsoft's Tuva Project. Full disclosure: I've only watched two so far; they were awesome. (Not really TCS, but I had to start with these.)

Here is another wonderful set of lectures by Erik Demaine. The course is called Planar Graph Algorithms and Beyond and is being taught by Erik Demaine, Shay Mozes, Christian Sommer and Siamak Tazari. They are also using some sections of Phil Kliens draft on Planar Graphs. I feel certain that it will make a great book after having seen the first two lectures.

Quantiki has a nice video abstract site, where people are free to post a short video (3-6 minutes) describing their recent papers/preprints. As it is part of Quantiki, it largely focuses on quantum information/computation.

I enjoyed Scott Aaronson's lecture at Caltech titled "Quantum Computing and the Limits of the Efficiently Computable." This lecture, which was in honor of Feynman, rehashes what's common knowledge to users of this site but in a very clear and funny way.

The lectures are great as you'd expect. But what really stands out to me is the production values: Each lecture's web page has embedded slides and lecture notes that play along in synchrony with the video. It uses high-definition HTML5 video with all the synchronization of slides and lecture notes done in JavaScript. Hopefully this will set a new standard of excellence.